[lug] replacing login shell

[D. Stimits]

Hugh Brown wrote:
> 
> On Wed, 2002-06-26 at 16:24, D. Stimits wrote:
> > One thing many people don't seem to realize (not talking about BLUG, but
> > unrelated experiences) is that a shell can be almost any program that
> > accepts stdin. If you look in /etc/, files "shells" and "passwd", you
> > can add different programs to become a login. A very long time ago, I
> > once added a MUD as the login shell to a few users I wanted to chat with
> > (and you could do this with an IRC client as well). If you were to get
> > the source to the ssh client, and hard wire it to a specific IP address,
> > and possibly disable a few things, you'd be much more secure than with a
> > script that can be suspended (let's say you have the script secure, then
> > you would still have the ssh problems, so having only ssh is not a
> > penalty compared to script controlled ssh). One thing that makes me
> > suggest hard waring is that you need to pass arguments to ssh client...I
> > would hard code it as needed, make it not accept arguments, and call it
> > something like "ssh-shell", then add it to /etc/shells, and alter given
> > login names to have this as the default shell (and if this is the case,
> > they will find it difficult to chsh to a non-ssh shell).
> >
> > D. Stimits, stimits at idcomm.com
> 
> I realize this is a general comment, but I am wondering what ssh
> problems there could be, in this specific case.  I suspend, but I don't
> get anything but another telnet prompt, I can't run chsh.  I want to
> know if there is some way of bypassing my set up, but I can't find one
> (I'm probably just ignorant).
> 
> Hugh

I have never tried, but if you use a shell script, it should generally
be able to do things with environment variables, or to simply kill it
before the shell gets far enough along, and have it do some alternative
behavior. Unless it is some form of a secure shell, perhaps in a chroot
environment, I would not consider it moderately secure (it should fool
the average guy, but not necessarily someone who knows something about
shells). Having one less program in the path of dominoes is a security
bonus.

D. Stimits, stimits at idcomm.com